Embodiments are described herein that relate generally to medical devices for treating various cardiac, physiologic and neurologic disorders. More particularly, embodiments are described that relate to implantable or external medical devices with a high voltage delivery circuit.
Numerous medical devices exist today, including but not limited to electrocardiographs (“ECGs”), electroencephalographs (“EEGs”), squid magnetometers, implantable pacemakers, implantable cardioverter-defibrillators (“ICDs”), neurostimulators, electrophysiology (“EP”) mapping and radio frequency (“RF”) ablation systems, and the like (hereafter generally “implantable medical devices” or “IMDs”). IMDs commonly employ one or more leads with electrodes that either receive or deliver voltage, current or other electromagnetic pulses (generally “energy”) from or to an organ or tissue (collectively hereafter “tissue”) for diagnostic or therapeutic purposes.
Certain types of IMDs include internal charge storage members, such as one or more capacitors. The charge storage members are connected to a switch circuit or network also referred to as an H-bridge. Conventional high voltage H-bridges include a network of transistors that are controlled to open and close in different combinations to deliver stored energy from the charge storage members to a patient through the electrodes. Heretofore, the H-bridge circuits in IMDs have used switches implemented through IGBTs (Insulated Gate Bipolar Transistors). An IGBT is a three-terminal power semiconductor device. However, IGBTs are relatively large and somewhat expensive.
Another type of switch device used in other electronic fields is a Silicon Controlled Rectifier (SCR). SCRs are smaller in size and less expensive than IGBTs. However, SCRs exhibit different operational characteristics than IGBTs. SCRs are latching devices, and thus once triggered an SCR switch will stay ON as long as current is flowing through the SCR. In other words, once an SCR switch is turned ON, it stays in the ON state while current is flowing. In the case of a high voltage delivery circuit, the voltage potential across an SCR switch exceeds the operating voltage of the switch's control circuit, Therefore, in a conventional H-bridge circuit if one of the SCR switches triggers before the other, a high voltage potential is created across the cathode of the un-triggered SCR switch. This high voltage potential exceeds the maximum operating voltage of the control circuit. Hence, the control circuit cannot generate enough voltage to drive current into the gate of the un-triggered SCR. Therefore, the control circuit fails to close the un-triggered SCR switch. Therefore, if a high voltage potential is created across an SCR switch while in an OFF state, the SCR switch cannot be changed to an ON state, Thus, an SCR switch exhibits more limited operational control as compared to other types of switches such as IGBT switches.
SCR switches are not readily substituted for IGBT switches in a high voltage H-bridge circuit, because the bridge circuit experiences certain operational difficulties when SCR switches are implemented. In many IMDs today, the high voltage bridge circuit includes three output terminals that are configured to be coupled to three separate electrodes capable of delivering high voltage energy to a patient. A network of six IGBT switches connects the output terminals to a high voltage positive (HVP) source and a high voltage negative (HVN) source. Each output terminal is located between, and in series with, corresponding pair of IGBT switches that are located between the HVP and HVN sources. One of each pair of IGBT switches open and close to connect or disconnect the corresponding output terminal, to one of the HVP and HVN sources.
SCR switches cannot be directly substituted for IGBT switches into a traditional H-bridge architecture because the latching behavior characteristic of the SCR switches adds a design complexity. For example, if a pair of output terminals are to be connected in parallel to the HVP sources, the risk exists that the SCR switches for one of the output terminal pair turns ON before the SCR switch for the second of the output terminal pair. When the first SCR switch turns ON, current begins to flow to the patient, thereby creating a voltage potential higher than the maximum voltage of the control circuit at the cathode of the second SCR switch. Once a voltage potential is created on the cathode of the second SCR switch, it stays in its initial state, namely OFF. Hence, the control circuit is not able to turn ON the second SCR switch and one of the two output terminals does not deliver a high energy shock.
Instead, the SCR switches should be opened simultaneously. However, opening the SCR switches simultaneously is not practical given the operational tolerances of the SCR devices and surrounding components.
Accordingly, a need remains for an improved high voltage H-bridge circuit that is able to realize the benefits of SCR switches without introducing the risks associated with the latching behavior of SCR switches.